Various devices or members such as honeycomb structures formed of ceramic superior in resistances to heat and corrosion have been used as a catalyst carrier for purifying an exhaust gas from viewpoints of environmental countermeasures such as atmosphere pollution prevention and global warming prevention in various fields including chemistry, electric power, iron and steel, and industrial waste disposal. As a method of manufacturing the honeycomb structure, for example, a method of manufacturing a honeycomb structure has been disclosed in which a cordierite forming material, water, organic binder and the like are kneaded, and a forming material whose plasticity has been enhanced is extruded/formed, dried, and fired (see, e.g., Japanese Patent Application Laid-Open No. 2002-292616).
The organic binder imparts plasticity or shape retaining property in order to enhance a forming property of the honeycomb structure. When an added amount increases, the forming property is enhanced. To form a large-sized structure or a structure having a complicated cell structure which has been increasingly demanded in recent years, kneaded clay (clay) having a satisfactory forming property is required as compared with a case where a small-sized or simple honeycomb structure is manufactured. As a result, a large amount of organic binder has to be added.
However, when the added amount of the organic binder is large, the organic binder is burnt out during firing. Therefore, there has been a problem that a space occupied by the organic binder becomes a defect at a forming time, and mechanical strength of the structure drops. In a large-sized structure, there has been a problem that the inside of the structure is at high temperature by burning heat when burning the organic binder during the firing, defects such as cracks are generated because of thermal stress by an inner/outer temperature difference of the structure, the mechanical strength of the structure is lowered, and yield is largely lowered. Furthermore, CO2 or harmful gas is generated, and released to the atmosphere by the burning of the organic binder during the firing, and this has raised a problem in environmental respects such as pollution and global warming.
Moreover, there have been disclosed: a method (see Japanese Patent Publication No. 2002-537217) in which kaolinite clay, and 0.1% to 15% by mass of smectite clay with respect to a dry mass of the kaolinite clay are used; synthetic clay for ceramic (Japanese Patent No. 3215839) comprising 30 to 65% by mass of amorphous silica, 30 to 65% by mass of alumina trihydrate, and 2 to 20% by mass of a mixture of one or two of sepiolite, palygorskite, and bentonite; a plasticity enhancing method (see Japanese Patent Publication No. 6-104563) of kaolinite particles, in which the kaolinite particles are brought into contact with an aqueous solution containing bivalent cations to thereby adsorb the bivalent cations onto kaolinite particle surfaces; a method in which smectite is added as a measure for enhancement of plasticity of clay or kaolin (Artificial Clay (see pages 175 to 178 of 10th Anniversary Journal of Artificial Clay Research Institute); and a method in which montmorillonite and hectorite are added to thereby enhance strength of a formed article in extrusion forming of alumina (see the Use of Montmorillonites as Extrusion Aids for Alumina, Ceram. Engi. Sci. Proc. 12 [1-2] pp. 33 to 48 (1991)).
However, as disclosed in the above-described patent or non-patent documents, main components of a clay mineral having plasticity, such as bentonite and smectite are SiO2, Al2O3, MgO and the like, and the mineral has sodium ions (Na+) and calcium ions (Ca2+) as interlayer cations. With respect to cordierite which is a main component of the finally obtained honeycomb structure, Na and Ca are impurities. Therefore, there has been a problem that low thermal expansion as one of characteristics of cordierite is inhibited, when a large amount of these impurities are contained.
The present invention has been developed in view of the above-described problem, and an object thereof is to provide a method of manufacturing a honeycomb structure, capable of preventing or reducing generation of CO2 or harmful gas during firing to thereby prevent or inhibit pollution and global warming, and capable of obtaining a honeycomb structure having few defects such as cracks and maintaining high strength and low thermal expansion, and a high-quality honeycomb structure obtained by this manufacturing method.